Method and apparatus for cleaning heat exchange apparatus connected to receive flue gases from soda recovery-boilers



Aprll 4, 1961 N; R. R. HULT ETAL 2,978,378

METHOD AND APPARATUS FOR CLEANING HEAT EXCHANGE APPARATUS CONNECTED TO RECEIVE FLUE GASES FROM SODA RECOVERY-BOILERS Filed Nov. 1a, 1958 s9 43 51 1819 I /4s 10 1 5 06 $1M i r 47 I I I 30: I

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I l l 5a I II I I I I I I I I I l it d. S a es e e METHOD AND APPARATUS FOR CLEANING .HEAT EXCHANGE APPARATUS CONNECTED TO RECEIVE FLUE GASES FROM SODA RECOV- 5 ERY-BOILERS Filed Nov. 18, 1958, Ser. No. 774,649 3 Claims. (Cl. 162-30) The present invention relates to soda recovery-boilers for the sulphate pulp industry, and is more particularly concerned with the provision of improved method and apparatus for cleaning during operation the gas-swept heat-transmission surfaces of heat exchange apparatus connected to receive from the boiler the flue gases generated by combusting the black liquor therein, and adapted to transmit to a fluid, such as the supply of feed water to the boiler, a substantial proportion of the sensible heat carried by the flue gases.

The flue gases from soda recovery-boilers are heavily laden with dust of a kind liable to form sticky and hard-to-remove deposits upon the gas-swept heat-transmission surfaces of the heat exchange apparatus. Such deposits will seriously impair the heat transmission to the receiving fluid, and may even prevent further operation of the heat exchange apparatus, if allowed to build up during any substantial length of time. By far the best way of keeping clean the gas-swept surfaces of the heat exchanger conceived until now is the method of introducing and scattering steel shot above the top of the heat exchanger, and to allow these shot to drop against the heat-transmission surfaces. During their travel downwards through the heat exchanger the shot will cascade over the said surfaces, and by their impacts most of the deposits adhering thereto will be shattered and broken away, and will be carried'away by the flue gases which travel downwards through the heat exchanger. This cleaning may be effected continuously or intermittently, and does not affect adversely the normal operation of .the heat exchange apparatus.-

Very far from being restricted to soda recovery-boilers the cleaning during operation by steel shot dropping against the heat-transmission surfaces of heat exchange apparatus has been adopted for almost every kind of plant wherein fuel is combusted, and it may be said that the cleaning of heat-transmission surfaces has generally ceased to be a major problem after the installation of such steel-shot cleaning.

In heat exchange apparatus connected to receive the flue gases from soda recovery-boilers, however, there exist some conditions having an adverse effect upon the efliciency of the steel-shot cleaning. a

The flue gases from the soda recovery-boiler hav a high percentage of moisture, and consequently precipitates will form on the steel shot travelling in the flue gas flow through the heat exchanger. The moist surfaces of the shot will readily pick up and retain the dust suspendedin the flue gases, and such dust will build up deposits on the steel shot which will impair the cleaning properties of the latter. The deposits formed upon the heat-transmission surfaces of the heat exchange apparatus as well as those formed on the steel shot are of a particularly tough and persistent kind, and the steel shot cleaning will not operate as efliciently as it will do in plants where the flue gases and thedust suspended therein will be of a less vdifiicult kind.

It is a chief object of the present irwentionto pro- 2,978,378 Patented v Apr. 4, 961

vide improved methods and apparatus increasing the efficiency of steel shot cleaning of heat-transmission sur-, faces in heat exchange apparatus connected to receive flue gases from soda recovery-boilers, by introducing in the flue gas stream after the soda recovery-boiler and before the heautransmission surfaces of the heat exchange apparatus particles apt to improve the cleaning action of the steel shot.

It is a further object of the invention to provide particles for the said purpose which are also apt to prevent the forming of deposits upon the heat transmission surfaces and other interior surfaces of the heat exchange apparatus.

It is a still further object of the invention to provide particles for the said purposes which are also calculated to replace losses of chemicals in the pulp-cooking process being served by the soda recovery-boiler.

Further objects, advantages and features will become apparent from the following description, reference being had therein to the appended diagrammatical drawing which is an elevation view of a soda recovery-boiler provided with improved and new apparatus for carrying the improved method into practice.

Before a description of the specific new improvements is attempted we will give a brief account of the apparatus illustrated in the drawing and its operation as far as already known in the art. This account will not be unduly detailed, however, since the operation of soda recovery boilers in the sulphate pulp industry is common knowledge to those skilled in the art, and in any case may be readily understood from any text-book on this subject. 7

The soda recovery-boiler is generally denoted 10. In operation it is fired with the combustible ingredients of the black liquor, i.e. the liquid residue formed after cooking the pulpwood in the digesters with white liquor, filtering away the pulp and evaporating most of the water from the liquor.

In the soda recovery-boiler water tubes (not illustrated) and drums 11, 13 are arranged in the usual manner, and the combustion gases are led in a serpentine path by means of screens such as 12. In the bottom rear part of the boiler a funnel-shaped hopper 14 is provided, and a flue from the top portion of the boiler is formed by another funnel-shaped hopper member 18.

The hoppers 14 and 18 are provided to separate some of the ashes entrained by the flue gases, especially those of a comparatively great particle size. Further, when blowing the heating surfaces with compressed air or steam thedeposits broken away may form sizable pieces, and it is important that such larger particles should not penetrate into the economiser 20 which is connected to the flue 18 by means of an angle-bent flue gas conduit 19, as the economiser tubes are heavily ripped and so closely adjacent that larger particles or lumps would be caught by the tubes and obstruct the gas passages therebetween.

The economiser 20 is provided at its bottom end with a funnel-like hopper 21 and a lateral flue conduit 23, and the flue gases passing downwards through the economiser between the tubes pass an inclined baflle member 22, oneobject of which is to increase the flue gas velocity before it enters into the conduit 23 so that dust and small particles suspended in the gas will be carried along into the flue in spite of the abrupt change of the gas flow direction.

Instead of an economiser 20 an air preheater or an aggregate comprising an air preheater as well as an economiser may be provided. For convenience we will refer in the continued description to an economiser.

The flue gases are sucked through the flue conduit 23 acre-ere by a fan 24, and this fan delivers into a conduit 25. The latter is connected to an electrostatic precipitator 26 in which the particles entrained by the flue gases are collected, and after that the clean flue gases pass through a conduit 27 into the stack (not illustrated).

The ashes collected inthe hoppers 14 and 18 may be fed to a fan 16 through conduits 15 and 17, respectively, and this fan delivers through a' pipe 28 into the conduit 25. If necessary or desirable, the ashes may be disintegrated by means of suitable apparatus (not illustrated) before being fed to the fan 16. Below hoppers 14 and 18 shutter means 29 and 30, respectively, are provided which may be used to close the hopper outlets when the fan 16 is not operated. By the arrangement now described the ashes caught in the hoppers 14 and 18 are by-passed the economiser and conveyed directly to the:

conduit 25 and precipitator 26 Where they are collected as well as the particles having passed through the economiser.

The black liquor to be combusted in the boiler is introduced into the boiler furnace through a pipe 31 which delivers the black liquor as a jet against an inclined plate member 32 by which it is broken up to form a finely-divided spray. The combustible constituents of the black liquor are subsequently combusted, and the remaining moisture is evaporated, and the combustion products follow the'path described above. The Na CO and Na s in the black liquor, on the other hand, will form a residual melt at the bottom of the boiler furnace, and this melt flows off through a pipe 40 and is delivered into a dissolving-tank 41 in which it is treated with a weak lye. The formed green liquor is pumped from the tank 41 through a pipe 42 and is treated subsequently with milk of lime to convert the sodium carbonate in the green liquor to sodium hydroxide for use in making up new white liquor for the digeste'rs.

Although most of the chemicals used for the process are recovered a portion gets lost and must be replaced. Beside the black liquor, therefore, salt-cake, Na SO is supplied to the recovery-boiler to make up for losses. The salt-cake is stored in a container 37 from which the salt crystals are metered by a feeding device 38 to a pipe 39 delivering into a mixing tank 35 which also receives the black liquor through another pipe 36. The mixture is pumped from tank 35 through a pipe 34 by a pump 33 which feeds the pipe 31 extending into the boiler furnace. The salt-cake after having reacted in well-known manner in the boiler furnace will join the melt at the furnace bottom and replace losses.

The particles collected in the precipitator 26 are also fed to the mixing tank 35, and for that purpose the following arrangement is made:

A valve device 63 connected to the collecting chamber of the precipitator is joined to an open-ended pipe 64 connected to a cyclone separator 65. The latter by an interposed hopper 66 delivers into a pipe 67 which extends down into the mixing tank 35. A suction pipe 68 joined to the top of cyclone separator 65 is connected to the inlet of a fan 69 and another pipe 70 extends from the fan outlet into the conduit 25 upstream of the pre cipitator 26. The precipitated particles are fed to the mixing tank 35 through members 63, 64, 65, 66, and 67, and the conveying air with such particles as may still be suspended therein is returned to the precipitator through pipe 68, fan 69 and pipe 70.

When passing the heat-transmission surfaces of the economiser 20 particles suspended in the flue gases from the recovery-boiler it) will build up deposits on the said surfaces which are apt to impair the heat transmission, and if not removed will prevent the operation of the economiser after a certain length of time. An arrangement for cleaning the economiser tubes therefore is essential. In the last few years cleaning of the heattransmission surfaces by means of steel shot which are allowed to drop by gravity from the top of the heat exchanger; are collected at'its bottom and recycled; has

replaced the previous methods of blowing with steam or compressed air or washing with an alkaline solution, since the steel shot cleaning is more eflicient and has the further advantage that the cleaning can be easily effected during operation of the heat exchange apparatus.

In the drawing there is illustrated a cleaning arrangementof this kind. Above the top of the economiser 20 a steel shot collecting and deflecting device 43 is disposed, and a pipe 44 extends from its bottom end into the conduit 19, and is laterally connectedto a retarding box 45 from the bottom of which another tube 46 extends vertically downwards and terminates a small distance above a: distributor member 47 which may be of the kind described and illustrated in U.S. patent specification No. 2,809,018 to Broman.

Below the bottom of the economiser 20 a collecting and separating device 48 is provided, and an inclined pipe 49 extends from a bottom outlet of device 48 to the lowermost portion of a straight vertically extending pipe 50, the bottom end of which is open. Pipe 50 extends to the level of the collecting and deflecting device 43, and penetrates into a chute member 51 laterally connected to the body of device 43. The upper end of pipe 50 (not illustrated) is likewise open and faces a top cover 52 on chute 51, said cover preferably being bolstered with a layer of relatively soft wearresistant material at its underside. From the top of the device 43 a pipe 53 extends to the inlet of a fan 54 which delivers into another pipe 55 connected to the flue conduit 25.

This cleaning arrangement may be operated continuously or intermittently, and it functions in the following manner.

Let it be assumed that a quantity of steel shot is stored in the collecting and deflecting device 43. These shot successivelypass' through pipe 44 into retarding box 45 wherein their fall is broken, and further down the pipe 46, and hit the distributor member 47. By rebounding against the member 47 the steel shot are scattered in all directions and drop against the topmost tubes in the economiser 20, and cascade from those tubes over the subsequent rows of tubes until they reach the hopper 21, and are collected in the collecting and separating device 48. The baflie member 22 prevents the possibility of their being intercepted by the mouth of flue conduit 23.

During their downward travel through the economiser the steel shot remove by their impacts against the heattransmission surfaces deposits formed on the latter, and depending upon their size those deposits either are entrained by the flue gases through conduit 23, or are delivered into the collecting and separating device 48. The steel shot and such collected particles are fed into the pipe 49, slide down the latter and enter the vertical pipe 50 laterally. The fan 54 sucks air from the device 43 through pipe 53, and thus induces a strong air flow in pipe 50 and chute 51. The air stream in pipe 50 conveys the steel shot and other particles into chute 51, and after having hit the bolstered cover 52 the shot and particles slide down chute 51 into the device 43. The steel shot by gravity drop to the bottom of device 43 and are subsequently fed to the distributor member 47, while at least part of the accompanying particles originating from the deposits removed from the heat-transmission surfaces and being lighterthan the shot are entrained by the air current through pipe 53, fan 54, and pipe 55 into conduit 25 to be collected in the precipitator 26.

This method of cleaning the heat-transmission surfaces of economisers and similar heat exchange apparatus operates very well as a whole, and is by far the most efl'icient one conceived until now. There are, however, certain factors which exert an unfavorable influence upon the elficiency of the cleaning with steel shot of heat exchange apparatus connected to receive flue gases from soda recovery=boilers. Theconibu'stic'ngasesgenerated" by firing the black liquor in a soda recovery-boiler, andsubsequentlycom ducted through heat exchange apparatus contain a great proportion of evaporated moisture which tends to form precipitates upon the steel shot travelling downwards through the heat exchanger. The moisture film formed upon the'shot is apt to attract and retain dust particles suspended in the flue gases, and by that reasondeposits willbuild up on the shot which owing to the presence of sulphurous agents in the dust will be of a sticky and particularly tough nature, and which are very hard to remove. Deposits of a similar persistent nature will also form on the heat-transmission surfaces. Since the cleaning action largely depends upon the hardnessand in regular shape of the shot which should have points and edges apt to break up the deposits on the heat-transmission surfaces, deposits formed upon the shot will impair their cleaning efiiciency, since such points and edges will be bolstered over, and the shot will assume a smoother surface and a more rounded shape. Due to that fact and also to the fact that the deposit layers formed on the shot surfaces have a hardness inferior to that of shot having no such deposits the cleaning action of the shot when impacting the heat-transmission surfaces will be lessened. That is the more unfortunate as the full cleaning efliciency of the'steel shot is of particular importance Where the deposits formed on the heat-transmission surfaces areof such a resistant kind as that encountered in heat exchange apparatus receiving the flue gases from soda-recovery-boilers. V V a -,Since the steel shot will be subjected to wear'upon their repeated recycling and impacts against the heattransmission surfaces their cleaning action will in'any case be impaired gradually, as their attacking edges and points are dulled. It is, therefore, the more important that the adverse effect of such wear upon the shot is not further amplified by allowing deposits to form upon the surface of the shot, since the compound shot will have a smaller specific weight than clean shot, and therefore will assume a smaller velocity prior to their impacts against the heat-transmission surfaces and consequently will attack the latter less effiicently than shot having no deposits.

Measures to keep the shot clean and free from deposits to enable them to attack the heat-transmission surfaces With full efficiency have therefore to be devised.

A solution of the problem of cleaning with a very high degree of eificiency heat-transmission surfaces in heat exchange apparatus after soda recovery-boilers is now oflered by our invention, and the improvements conceived by us will be described presently.

Primarily our invention comprises the step of introducing into the flue gas stream after the soda recoveryboiler and before the heat exchange apparatus particles of a kind apt to prevent the forming of deposits upon the steel shot, the heat-transmission surfaces of the heat exchange apparatus and the interior casing walls and flue gas inlet of the latter.

Secondarily our invention comprises the step of introducing for the said purpose particles of a kind calculated to replace losses of chemicals in the sulphate pulp circuit served by the soda recovery-boiler.

We have found that the introduction of salt-cake crystals into the top of the economiser (or air preheater or a combination of both) will prevent the forming of deposits upon the shot which will therefore retain to the fullest extent their cleaning efliciency.

We have found, as well, that the introduction of saltcake crystals has a lessening effect upon the building-up of deposits on the heat-transmission surfaces, the interior casing walls of the heat exchanger and the flue gas in let to the latter.

We have drawn the conclusion that those results may possibly be due to some electro-static action caused by the sat-cake crystals, and it may also be, that the latter, which, although quite small, are much larger than the 61 deposit-building dust particles in the-flue gases, form nuclei upon which the said particles are precipitated and carried away without having'had the opportunity to forni deposits on the steel shot and on the heat-transmission surfaces and other surfaces within the heat exchanger.

We refer again tothe drawing wherein there is illustrated an inclined chute 56 into which the feed device 38 from container 37 is made to deliver part of the salt-cake crystals metered out. This chute 56'is given an oscillatory movement by a vibrator 57 which is driven by suitable drive means (not illustrated). The chute 56 delivers into a funnel-shaped tube member 58 which opens into a pipe 59, one end of which is connected to the outlet of a fan 60 and the other end of which penetrates a short distance into the flue conduit 19 at the top of the economiser 20. 'Beyond the end of pipe 59 and Within the conduit 19 an inclined plate member 61 is disposed, and the salt-cake crystals conveyed through pipe 59 by the fan 60 are scattered in all directions upon hitting plate member 61 so qthat'they are mixed wth the flue gases and travel downwards the same path as the latter. Any'salt-cake dust which is stirred up at the outlet of the chute 56 and mouth of funnel 58 is caught by asuction tube 62 connected to the inlet of fan 60, and also conveyed through pipe 59 to theeconomiser 20.

The salt-cake crystals used for cleaning purposes are entrained by the fluegases through conduits 23 and 25 to the precipitator 26 where they are collected together with other particles and fed to the mixing tank 35, as already described. They consequently are recovered, and serve to make up for lossesas well as the salt-cake fed directly into mixing tank 35through pipe 39.

The relative quantities of salt-cake to be fed directly to the mixing tank and to the flue gas stream before the heattransmission surfaces, respectively, may vary owing to the particular conditions in different plants. We have found that 30 to 40% of the total supply of salt-cake may advantageously be fed to the flue gas stream.

Above the unexpected results obtained with the heating surface cleaning method now described we have found that even if the steel shot cleaning is not operated the introduction of salt-cake crystals into the heat exchanger is etfective to keep the heat-transmission surfaces and other interior surfaces of the heat exchanger and the flue gas inlet thereto reasonably clean and free from deposits for shorter periods of times This result is consistent with the assumption that the salt-cake crystals form nuclei upon which a substantial proportion of the dust suspended in the flue gases will settle and be conveyed from the heat exchange apparatus without having had the opportunity to form deposits upon the heat-transmission surfaces and other surfaces referred to above. This is a further obvious advantage obtained with our improved cleaning method, since, if the steel shot cleaning system should fail temporarily, i.e. through break-down of the fan 54 or by any other reason, the operation of the boiler need not be discontinued, and the respite gained will in most cases be sufficient for effecting the necessary repairs of the steel shot cleaning system.

Likewise, a break-down of the system feeding salt-cake crystals to the economiser will not have any immediate serious consequences since the steel shot cleaning system continues to operate. In such case it may only be necessary to increase the quantity of salt-cake crystals fed directly to mixing tank 35 to compensate for the temporary shutting-01f of the supply of salt-cake crystals to the economiser 20. Further, it is preferable that the arrangement for feeding salt-cake crystals should make it easily possible to vary the total rate of supply of salt-cake crystals as well as of the proportions fed into mixing tank 35 and pipe 59, respectively, and suitable arrangements therefor will be obvious to those skilled in the ant.

Our invention thus not only provides a more eificient cleaning of heat exchange surfaces in soda recoveryaerasas boilers, but al's'oimproves the reliability ot theheat-transm'ission surface cleaning system as such. 7 3

It should be clear that changes and modifications of the method and apparatus described and illustrated may be made without departing from the spirit of invention, the scope of which should be restricted only'by the appended claims. V

What we claim is: I 1. In the operation of a soda recovery-boiler from which dust-laden flue gases generated by combusting black liquor in the boiler furnace are conducted through heat exchange apparatus the gas-swept heat-transmission surfaces of which are cleaned during operation by introducing steel shot into a space above said heat exchange apparatus, said steel shot being scattered andallowed to drop against said heat-transmission surfaces to remove therefrom deposits formed thereon by the dust upon the passage of said flue gases through said heat exchange apparatus, the step which comprises adding to the flue gas stream after said boiler and before said heat exchange apparatus particles of salt-cake. v

2. In the operation of a soda recovery-boiler from which dust-laden flue gases generated by combusting black liquor in the boiler furnace are conducted through heat exchange apparatus the gas-swept heat-transmission surfaees of which are cleaned during operation by introducing steel shot into a space above said heat exchange apparatus, said steel shot being scattered and allowed to drop against said heat-transmission surfaces to remove therefrom deposits formed thereon by the dust upon the passage of said flue gases through said heat exchange apparatus, the steps which comprise adding to? the flue 8 gas stream' after said boiler audbefore said heat exeharige apparatus particles of s'alt cake, recovering said particlesfrom said gas stream after said heat exchange apparatus, and mixing said recoveredparticles with said black liquor.,

3. In the operationof a soda recovery-boiler from which dus't laden flue gases'g'euerated by combusting black liquor in the boiler furnace are conducted through heat exchange apparatusthe gas-swept heat transmission surfaces of whichare cleaned during operation'b'y introducing steel shot into a space above said heat exchange apparatus' saidsteel' shot being scattere'd and allowed to drop against said heat-transmission surfaces to remove therefrom deposits formed thereon by the dust upon the passage of said'flue' g'ases'through said heat exchange apparatus, in which operation salt-cake is mixed with said black liquor prior to combustion to make up the loss of chemicals during said operation, the steps which com prise introducing from 30% to 40% of the total makeup salt-cakei n particulate form directly into the stream of fluegas after said boiler and before said heat exchange apparatus, and recovering said particles from said gas stream after said heat exchange apparatus.

References Cited in the file of this patent UNITED STATES- PATENTS 1,670,723 Clarkson Mar. 2,

S'avell Sept. 12, 1244 

1. IN THE OPERATION OF A SODA RECOVERY-BOILER FROM WHICH DUST-LADEN FLUE GASES GENERATED BY COMBUSTION BLACK LIQUOR IN THE BOILER FURNANCE ARE CONDUCTEC THROUGH HEAT EXCHANGE APPARATUS THE GAS-SWEPT HEAT-TRANSMISSION SURFACES OF WHICH ARE CLEANED DURING OPERATION BY INTRODUCING STEEL SHOT INTO A SPACE ABOVE SAID HEAT EXCHANGE APPARATUS, SAID STEEL SHOT BEING SCATTERED AND ALLOWED TO DROP AGAINST SAID HEAT-TRANSMISSION SURFACES TO REMOVE THEREFROM DEPOSITS FORMED THEREON BY THE DUST UPON THE PASSAGE OF SAID FLUE GASES THROUGH SAID HEAT EXCHANGE APPARATUS, THE STEP WHICH COMPRISES ADDING TO THE FLUE GAS STREAM AFTER SAID BOILER AND BEFORE SAID HEAT EXCHANGE APPARATUS PARTICLES OF SALT-CAKE. 